Mechanisms of important cell-to-cell fusion processes, including sperm-egg fusion in fertilization, trophoblast fusion in placentogenesis, myoblast fusion in skeletal muscle development and macrophage fusion in osteoclast formation, remain to be understood. In our recent projects we explored mechanisms of mammalian myoblast fusion. Understanding myoblast fusion mechanisms is important for developing new therapies for acceleration of muscle regeneration and may bring useful insights into cancer cachexia, some myopathies and muscular dystrophies. Using different assays, we isolate the actual event of merger of two plasma membranes from processes that prepare the cells for fusion and from processes that underlie the transition from early fusion connections to syncytium formation. We have focused on a potential role of myoblast fusion defects in spinal muscular atrophy (SMA), a hereditary disease that causes motor neuron degeneration and general muscle wasting. SMA, the most frequent inherited cause of infant mortality and the second most common fatal autosomal recessive disorder, affects 1 in 10,000 people, mostly children. The mechanisms by which mutations in the gene encoding survival motor neuron protein (SMN) cause progressive muscle atrophy remain elusive and several studies have suggested a role for muscle and, specifically, myoblast fusion deficiency in the pathophysiology of SMA. Research on cultured cells and animal models suggests that SMN-deficiency in muscles develop a phenotype reminiscent of that observed in mouse models of muscular dystrophy suggesting intrinsic muscle defects. Moreover, a recent study has demonstrated that rescuing SMN in muscle alone improves the lifespan of SMA model mice suggesting that skeletal muscles represent important targets for therapeutic intervention in SMA. SMA muscle has been reported to have smaller myotubes suggesting a delay in myotube growth and maturation for SMN-deficient myoblasts suggesting possible defects in myoblast fusion. We investigated the role of SMN in muscle development using muscle cell lines and primary myoblasts and confirmed earlier reports that SMN-deficient myoblasts fuse much worse than wild type cells. Importantly, this fusion defect was due to reduced SMN levels and transfecting SMN-deficient cells to express SMN rescued the fusion to the levels approaching those of wild type myotubes. The local merger between myoblast plasma membranes and subsequent expansion of nascent fusion connections are two stages of myoblast fusion that are controlled by distinct protein machineries. To test whether SMN deficiency blocks myotube formation downstream from membrane merger, we co-plated myoblasts labeled with different membrane probes in differentiation. Merger of the membranes of differently labeled cells generates co-labeled cells. As expected, for wild type myoblasts, we observed the appearance of multinucleated and co-labeled cells. In contrast, in the case of SMN-deficient myoblasts, we observed neither syncytium formation nor co-labeled cells. This finding indicates that the lack of SMN inhibits myotube formation upstream of even the earliest stages of myoblast fusion. Based on our findings, we suggest that enhancement of early stages of impaired myoblast would attenuate fusion defect in SMN-deficient myoblasts and potentially slow down muscle atrophy in SMA. More generally, better understanging of the molecular mechanisms by which proteins shape membranes to initiate diverse fusion processes will help in developing new ways of controlling cell fusion throughout normal development of many human organs and tissues and in many genetic and infectious diseases.